A traffic monitoring system monitors a flow of vehicles (hereinafter referred to as “traffic flow”) traveling on a road. In the traffic monitoring system that uses sensor units such as loop coils, ultrasonic sensors, or the like, two or more sensor units are provided at different positions in order to measure a number and a velocity of the vehicles traveling on the road. By measuring times at which a vehicle passes two sensor units that are separated by a predetermined distance along a traveling direction of the vehicle, for example, an average velocity of the vehicle between the two sensor units can be obtained.
On the other hand, a traffic monitoring system that uses a scan type radar apparatus to detect the vehicles has also been proposed. For example, a FM-CW (Frequency Modulated-Continuous Wave) radar apparatus periodically changes a transmission wave frequency and continuous transmitting the transmission wave without a break. Because the frequency of the transmission wave is already changed by the time a reflected wave from a detection target is received, a difference between the frequencies of the transmission wave and the received reflected wave may be measured to measure a time delay of the reflected wave, in order to detect a distance from the single FM-CW radar apparatus to the detection target and a moving velocity of the detection target. For this reason, by using the FM-CW radar apparatus in the traffic monitoring apparatus and detecting the number and the velocity of the vehicles traveling in each lane of the road monitored by the FM-CW radar apparatus, it is possible to grasp the traffic (or degree of traffic jam) in each lane of the road, and detect foreign objects such as trash existing on each lane.
In a case in which a size of the traveling vehicle traveling is to be included in the information measured by the traffic monitoring system, and the sensor units such as the ultrasonic sensors described above are to be used, the configuration of the traffic monitoring system becomes complex because two or more sensor units need to be provided. On the other hand, in a case in which the radar apparatus is used for the traffic monitoring system, another apparatus or means needs to be provided separately in order to measure the size of the traveling vehicle because it is difficult to measure the size of the traveling vehicle by the radar apparatus. In this case, the configuration of the traffic monitoring system becomes complex due to the need to separately provide the apparatus or means for measuring the size of the traveling vehicle.
It is possible to measure the size of the traveling vehicle from an image picked up by a camera. However, because the size of the vehicle within the picked up image changes depending on the distance from the camera to the vehicle, another apparatus or means needs to be provided separately in order to measure the distance to the traveling vehicle. In this case, the configuration of the traffic monitoring system becomes complex due to the need to separately provide the apparatus or means for measuring the distance to the traveling vehicle.
Examples of conventional systems are described in Japanese Laid-Open Patent Publications No. 2011-257814 and No. 2011-017634, for example.
According to the conventional systems or techniques, it is difficult to measure the size of the traveling vehicle using a simple configuration.